Cam shafts for engines to date encompass a cam shaft including a shaft part that is solid (solid type) and a cam shaft including a shaft part that is hollow in an entire length direction of the shaft part (hollow type). Patent Literature 1 discloses one example of a method for manufacturing a cam shaft of the hollow type.
The method for manufacturing the cam shaft described in Patent Literature 1 includes the steps of: manufacturing a casting mold by placing, in a main mold that is made of sand and/or the like and is collapsible, a core that is made of sand and/or the like and is collapsible; pouring a molten metal into a cavity formed between the main mold and the core; and causing the casting mold to collapse and removing the casting mold after the molten metal is cooled. In addition, the core is provided with a plurality of positioning pins protruding from an outer circumferential surface of the core in a radial direction. The plurality of pins define the position of the core in the main mold.
According to the method for manufacturing the cam shaft described in Patent Literature 1, it is possible to manufacture a cam shaft having a lighter weight than a cam shaft of the solid type. However, the method of Patent Literature 1 may cause the following problems.
That is, according to this method, it is necessary to prepare a core for hollowing a shaft part of a cam shaft and to position the core in a main mold. Furthermore, in order to accurately define the position of the core in the main mold, the core needs to be provided with pins as those described above such that the pins are projected from the core. Consequently, there arise a problem that the number of casting mold parts increases and accordingly the manufacturing cost of the casting mold increases. Moreover, by shot blasting or the like, it is necessary to cause the core to collapse and to remove the core via a shaft hole of the cam shaft having been removed from the main mold. Consequently, there arises a problem that the number of steps for manufacturing the cam shaft is significantly increased as compared with the cam shaft of the solid type.